The present invention generally relates to solar cells for the generation of electrical power and, more specifically, to improved solar cells used with high solar flux solar cell concentrators, which are easily manufactured and generate high power at high efficiency.
The conversion of solar energy into electrical energy with photovoltaic cells is a major contributing source for the production of electrical power for public and private use, as costs associated with more traditional power plants, such as those employing conventional energy sources, increase yearly and environmental concerns restrict the construction of new generating plants. The solar energy generation of high power at high efficiencies ultimately results in reduction of most system costs, such as land acquisition and usage, support structures, operating manpower and wiring.
One method used in the past to increase efficiency is to manufacture solar cells with multiple layers having different energy band gaps stacked so that each cell or layer can absorb a different part of the wide energy distribution in the sunlight. Because of the cell""s high voltage and susceptibility to reverse bias breakdown, there is a requirement to protect each cell with a bypass diode. Attachment of the diode to each cell, in addition to attaching interconnects for the purpose of increasing voltage by series connection, increases complexity and manufacturing costs.
Also, in the past, connection of cells has involved multiple interconnects and diode tabs. The diode tabs have commonly been separate strips of metal, connecting the diode electrically from the front of the cell to the rear of the cell. This has required much handling, attaching, and cleaning, thus increasing manufacturing costs, as well as solar cell attrition due to handling.
Traditionally, once the individual solar cells have been interconnected in a string, the string is then bonded to a substrate. Wiring the cell strings together in series for higher voltage or in parallel for higher current has typically been accomplished by the use of metal tabs or wire and soldered or welded joints. However, this method of attachment involves a time consuming set of manual processes, which require inspection, rework and cleaning. Along with being time consuming, those steps also lead to attrition of the fragile and expensive solar cells.
Past designs of solar cell panels that attempt to address one or more of the above performance and manufacturing issues have been numerous. One design includes encapsulating solar cell modules in a polymer cover film molded to provide an embossed surface having depressions arranged in a row. Each depression has the same configuration as a solar cell. Solar cells with positive and negative contacts on the back surface are preferred and can be positioned in the depressions with the front surfaces of the cells that face the light source contacting the bottom of the depressions. A second polymer film having interconnecting circuitry metallization is placed over the back surfaces of the cells so that the cells are electrically connected. A disadvantage of the concept is the lack of direct bonding between the back surfaces of the cells and the second polymer film, which leads to a greater potential for separation from the metallization. Another disadvantage is that the device may not work in a severe thermal environment where thermal expansion may result in a loss of electrical connection due to coefficient of thermal expansion mismatch.
Another past design uses a printed circuit substrate whereby the solar cells are physically and electrically connected to a substrate via interconnect pads. Positive and negative terminals on the back side of the cells are connected by soldering to the interconnect pads. If the terminals are on opposite sides of the cells, metallic interconnectors are used to connect terminals on the tops sides, over the cell edges, and to the interconnect pads. An adhesive may be used to secure the cells to the substrate. Stress relief loops bound the interconnect pads to electrical traces encapsulated in the substrate. This results in the solar cells being effectively mounted to the substrate on coiled springs. On the back side of the substrate, electrically conductive mounting pads enable attachment to elements such as blocking and shunting diodes. If the cell is soldered to the spring shaped conductor then the solder could bridge across the spring, thus making it lose its advantage as an absorber of thermal stresses. Another disadvantage is that the configuration of a coiled loop provides a relatively weak structure that is susceptible to structural failure when stressed and, thus, electrical connection failure. Yet another disadvantage is that this design requires either a wrapthrough metal configuration to bring both cell contacts to the rear side of the solar cells or a tab. The tab type described in the patent bridges off the cell onto an adjacent conductive pad, which increases the area required for a solar array of a given power design. The wrapthrough metal configuration has the disadvantage of being very costly to manufacture because it requires a number of expensive photomasks and photoresist processes. Other disadvantages of prior art designs include: obscuration losses resulting from extensively sized interconnects and ohmic bars; poor thermal conduction resulting from use of overly thick di-electric adhesives to bond the solar cell assemblies to the heat sink; and high stresses resulting from mismatched Coefficients of Thermal Expansion (CTE) between the solar cells and their substrates.
As can be seen, there is a need for improved solar cell concentrator modules that are easily manufactured and generate high power at high efficiency. One method for increasing efficiency is to allow direct bonding of concentrator solar cells to a heat sink. This approach allows series or parallel interconnection between multiple cells and provides for high thermal conductance to improve cooling the solar cells. Cooling the solar cells under high concentration of solar energy increases their electrical efficiency by increasing their voltage. Another method for increasing efficiency involves utilization of a high thermally conductive di-electric to insulate the cell backs from the metal heat sink.
These and other objects, features and advantages of the present invention are specifically set forth in, or will become apparent from, the following detailed description of the embodiments of the invention when read in conjunction with the accompanying drawings.
In one aspect of the present invention, a solar cell concentrator receiver comprises a solar cell, an electrically conductive interconnect, and a di-electric element sandwiched between two metallic elements. The top metallic element has an etched electrical circuit and the rear metallic element has an etched pattern for thermal conduction. Electrically conductive adhesives or solder secure the solar cell and electrically conductive interconnect to the di-electric, and to a heat sink.
In another aspect of the present invention, a solar cell, insulator, conductor circuit comprises a solar cell, an electrically conductive interconnect, and a di-electric element sandwiched between two metallic sheets. One of the metallic sheets has an etched electrical circuit pattern and the other metallic sheet has an etched pattern for thermal conduction. Electrically conductive adhesives or solder secure the di-electric sheet element between the solar cell and a heat sink.
In yet another aspect of the present invention, an apparatus for the generation of electrical power from a high solar flux photovoltaic concentrator receiver comprises a solar cell with an electrically conductive interconnect leading from the top side to the rear side, and a di-electric sheet sandwiched between two metallic sheet elements. One metallic sheet element has an etched electrical circuit pattern and another metallic sheet element has an etched pattern for thermal conduction. Electrically conductive adhesives or solder secure the di-electric sheet element between the solar cell and the heat sink. The apparatus converts solar energy to electrical energy, and provides an electrical power transfer and interconnection path together with a thermally conductive path for heat dissipation.
In yet a further aspect of the present invention, a process is disclosed for manufacturing a solar cell concentrator receiver. The process involves the steps of installing an electrically conductive wraparound interconnect on one side of a solar cell, assembling the solar cell into an alignment fixture, etching patterns on front and back metal faces, applying conductive adhesive or solder to both sides of the di-electric element, bonding said di-electric element to the solar cell on one side and to a heat sink on the opposite side, curing the adhesives or cleaning off solder flux residue, and electrically and mechanically connecting the assembled solar cell concentrator to a substrate heat sink.
Other aspects, advantages and features of the invention will become more apparent and better understood, as will equivalent structures, which are intended to be covered herein, with the teaching of the principles of the invention in connection with the disclosure of the embodiments thereof in the specification, claims, and drawings.